I am running MOSS 2007 - pre SP1 on a client's Farm and I came across this error with the crawl logs "The docfile has been corrupted." for one of the documents.
I also noticed that the document and Excel 2003 file was not in the index and it didnt return any result if I perform a search for the document. When I googled it I came across a fix which I am still yet to test.
Here is the link Fix.
It is simply to save this excel file as a 2007 file, this should fix it.